1. Field of the Invention
The present invention relates to an array-type capacitive pressure pulse wave sensor for measuring a waveform indicating a change in arterial pressure, and a pulse wave measuring apparatus provided with the same.
2. Description of the Background Art
As a pressure pulse wave measuring method for obtaining a waveform indicating a change in arterial pressure in a noninvasive and simple manner, a tonometry method is known, which is described in G. L. Pressman, P. M. Newgard, “A Transducer for the Continuous External Measurement of Arterial Blood Pressure”, IEEE TRANSACTIONS ON BIO-MEDICAL ELECTRONICS, 1963, pp. 73–81 (hereinafter, referred to as “Publication 1”). According to the tonometry method, a flat plate is pressed against the surface of the living body to deform the underlying artery to a flattened form, and, with the surface of the artery being kept in the state where the influence of the tension is eliminated, only the change in arterial pressure is measured accurately and stably.
In recent years, attempts have been made to measure the states within the living body by obtaining characteristic values from the waveform indicating the change in arterial pressure measured by using the tonometry method. As one of such attempts, investigation of the AI (Augmentation Index) value as an index for determination of degree of hardening of the artery has been conducted vigorously.
Measurement of the waveform indicating the change in arterial pressure using the tonometry method requires, not only that the artery is flattened, but also that a sensor element is arranged directly above the flattened artery. Further, to conduct the measurement of the waveform indicating the change in arterial pressure with accuracy, it is necessary to ensure that the sensor element has a width smaller than the width of the flattened part of the artery. To this end, the sensor element needs to be sufficiently small compared to the diameter of the underlying artery. In view of the foregoing, since it is very difficult to position a single sensor element immediately above the flattened region of the artery, it is practical to use a pressure sensor having a plurality of microfabricated sensor elements arranged approximately orthogonal to the direction in which the artery extends, to measure the pressure pulse wave.
U.S. Pat. No. 4,269,193 (hereinafter, referred to as “Publication 2”) and Japanese Patent Laying-Open No. 63-293424 (hereinafter, “Publication 3”) disclose the pulse wave measuring apparatuses implementing the measurement principle described above. In each of the pressure pulse wave sensors disclosed in Publications 2 and 3, the sensor element has a width (of about 0.2 mm to 0.5 mm) that is sufficiently smaller than the diameter of the artery (normally on the order of 1.2 mm to 3.5 mm), and a large number of such miniaturized sensor elements are arranged in the direction approximately orthogonal to the extending direction of the artery, so that at least one sensor element is positioned directly above the flattened artery.
In the pulse wave measuring apparatus disclosed in Publication 2, as a pressure sensor satisfying sensitivity and S/N ratio of required levels, a semiconductor pressure sensor having a plurality of diaphragms formed in a monocrystalline silicon chip by anisotropic etching is described. Further, in the pulse wave measuring apparatus disclosed in Publication 3, use of a piezoelectric element, a semiconductor strain gage, or a pressure-sensitive diode or transistor formed on a semiconductor chip, as a pressure sensor is described. Pressure sensors utilizing such strain resistance elements are suitable for the pressure sensor satisfying the above-described conditions, since they can be miniaturized by applying a semiconductor manufacturing process or the like. Such miniaturization of the sensor element, however, inevitably increases the manufacturing cost to a large extent.
Generally, as the sensing technique for measuring pressure, the one utilizing a capacitive element is known, besides the one utilizing the strain resistance element. In the sensing technique utilizing the capacitive element, the sensor element has a relatively simple structure compared to that of the strain resistance element, which can be manufactured inexpensively without the need of using the semiconductor manufacturing process requiring a large manufacturing cost.
Although not intended to be used for obtaining a waveform indicating a change in arterial pressure, as a pressure sensor having capacitive elements arranged in an array on a sensing surface, tactile sensors are described in R. S. Fearing, “Tactile Sensing Mechanisms”, The International Journal of Robotics Research, June 1990, Vol. 9, No. 3, pp. 3–23 (hereinafter, “Publication 4”) and in D. A. Kontarinis et al., “A Tactile Shape Sensing and Display System for Teleoperated Manipulation”, IEEE International Conference on Robotics and Automation, 1995, pp. 641–646 (hereinafter, “Publication 5”).
Hereinafter, of the tactile sensors described in Publications 4 and 5, the one described in Publication 5 will be described in detail. FIG. 16 is a perspective view of a pressure detecting portion of a tactile sensor described in Publication 5, and FIG. 17 is an exploded perspective view of the pressure detecting portion shown in FIG. 16. FIG. 18A is a plan view of the pressure detecting portion of FIG. 16 when seen from the above, and FIG. 18B is a schematic diagram showing a layout of the capacitive elements in the pressure detecting portion of FIG. 16. FIG. 19 is a circuit configuration diagram of the tactile sensor including the pressure detecting portion shown in FIG. 16.
As shown in FIGS. 16 and 17, the tactile sensor 1E described in Publication 5 primarily includes lower electrodes 10, upper electrodes 20, and spacer members 30. Lower electrodes 10 are formed of a plurality of copper strips that are arranged side by side in rows to extend substantially linearly. Upper electrodes 20 are formed of a plurality of copper strips that are arranged side by side in columns to extend substantially linearly in a direction orthogonal to lower electrodes 10. Spacer members 30 formed of silicon rubber are arranged between lower electrodes 10 and upper electrodes 20.
At each of the intersections of lower electrodes 10 and upper electrodes 20 arranged in rows and columns, a part of lower electrode 10 and a part of upper electrode 20 face each other with a prescribed distance therebetween secured by spacer members 30. In this manner, capacitive elements 40 (see FIG. 18A) serving as the sensor elements are formed at the intersections.
As shown in FIGS. 18A and 18B, in tactile sensor 1E of the above-described configuration, capacitive elements 40 are aligned in the form of an array when the pressure detecting portion is seen in two dimensions. Each capacitive element 40 has its capacitance changed as pressure applied to upper electrode 20 or lower electrode 10 causes them to deflect in the direction decreasing the distance therebetween.
Now, with lower electrodes 10 and upper electrodes 20 arranged in rows and columns, assume a circuit configuration where one electrodes, i.e., the lower electrodes or the upper electrodes, are connected via a multiplexer 50 to a power source 60 and the other electrodes, i.e., the upper electrodes or the lower electrodes, are connected via multiplexer 50 to a detector 70, as shown in FIG. 19. With this configuration, when a particular lower electrode 10 and a particular upper electrode 20 are selected by means of multiplexer 50, capacitance of a specific one of the capacitive elements 40 arranged in the array form can be obtained via detector 70. For example, in FIG. 19, when lower electrode 10 on the second row from the top and upper electrode 20 on the third column from the left are selected, the capacitance of the capacitive element denoted by a reference character 41 is output. Thus, it is possible to measure the pressure at a given position on the sensing surface of tactile sensor 1E.
If the conventional pressure sensor of capacitive type as described above is used as the pressure pulse wave sensor for measuring the pressure pulse wave, the manufacturing cost will be considerably decreased compared to the case of the pressure sensor using the strain resistance elements described above. However, the capacitive type pressure sensor is inferior in terms of miniaturization compared to the pressure sensor using the strain resistance elements manufactured by the semiconductor process. The currently workable minimum width of the capacitive element is about 1.0 mm to 2.0 mm.
When the capacitive pressure sensor of the above-described structure is used as the pressure pulse wave sensor, misalignment between the central position of the sensor element and the central position of the artery will be A/2 at a maximum when the distance between the neighboring sensor elements is represented by A, as shown in FIG. 18B. Thus, the maximum amount of misalignment between the central position of the sensor element and the central position of the artery when the capacitive pressure sensor of the above structure manufactured with the currently workable minimum width dimension is used as the pressure pulse wave sensor will be about 0.5 mm to 1.0 mm. This is considerably inferior to the case where the pressure sensor of strain resistance type described above is used as the pressure pulse wave sensor. If such a capacitive pressure sensor is adapted as it is, there will occur a large error in the measured value, hindering measurement with high accuracy.